1. Field of the Invention:
This invention pertains to mixtures of polymethyl methacrylate (PMMA) and polycarbonates which contain at least 15 mole % of fluorinated bisphenol monomer units (F-PC) such as 2,2-bis-(4-hydroxyphenyl)-hexafluoropropane (6F-bisphenol A), herein referred to as F-PC. Blends of F-PC and PMMA are thermodynamically miscible, single phase systems which are transparent in all ratios.
2. Discussion of the Background:
Blends of polycarbonates and polymethyl methacrylates are known. Generally, polycarbonates exhibit properties such as high temperature stability, good dimensional stability, high impact strength, good stiffness and most notably good transparency. For these reasons, PC is used in a variety of applications including glass replacement, housings, medical devices and containers. Nevertheless, PC does have drawbacks such as poor scratch resistance, poor long-term U.V. resistance and stress birefringence which have to be dealt with, particularly in demanding optical applications.
Polymethyl methacrylates, on the other hand, are limited by their relatively poor dimensional stability, low impact strength and low temperature stability. They are, however, known for their clarity, surface hardness, U.V. resistance and generally good weatherability and chemical resistance. For this reason, they are extensively used in applications such as window glazings, aircraft windows and automotive lenses/ lightcovers.
Blends of PC and PMMA would therefore be expected to eliminate the individual deficiencies of the respective components and result in a material having considerably improved mechanical and optical properties for a wide range of applications. Unfortunately, blends of PC and typical polymethyl methacrylates are not thermodynamically miscible at most compositions and their mixtures result in opaque materials which are not acceptable for transparent, optical applications. The technical literature has previously debated the miscibility of these mixtures but the current understanding (for example see Polymer, Volume 32, page 272, 1991) shows that traditional, free-radically polymerized PMMA does not form a single, thermodynamically miscible, transparent blend but does demonstrate mechanical compatibility with PC. The term "thermodynamically miscible" refers to a polymer blend that is mixed on the molecular level so as to form a single homogeneous phase which exhibits only one glass transition. In contrast, the term "mechanically compatible" is taken to mean that mixing of the polymers is on a small scale but larger than the molecular level. Furthermore, "mechanically compatible" implies that the multiple phases exhibit good adhesion to one another so as to yield good mechanical properties. Although both thermodynamically miscible and mechanically compatible blends exhibit good mechanical properties, thermodynamically miscible blends will generally be stronger, and only thermodynamically miscible blends are transparent, owing to their single phase nature.
Many references now exist which describe mixtures of PC and PMMA as immiscible in most proportions. For example, U.S. Pat. No. 4,319,003 teaches that blends of PC and PMMA are opaque and do not possess the advantageous properties exhibited by either polymer. In Polymer Preprints, Volume 23, pages 258-259, 1982 and in Advances in Chemistry, Volume 206, Number 9, pages 129-148, 1984, indications of immiscibility in all typical PC/PMMA blends are reported; as a matter of fact, these blends were observed to be opaque over the entire composition range. Among several other references which confirm that these mixtures are immiscible ar JP 7216063 and EP 0297285.
Ways of overcoming the drawbacks associated with the immiscibility of typical PC/PMMA blends have been previously proposed. Among them, the addition of copolymer additives (DE 2264268); PMMA/acrylamide copolymers (DE 3632946); and PMMA-ester copolymers containing carboxylic groups (U.S. 4,906,696). In U.S. Pat. No. 4,319,003, the use of a block copolymer of PC and PMMA instead of a physical mixture of the two components is proposed.
Similarly, processes have been developed and proposed which can also produce transparent PC/PMMA blends. According to DE 3,833,218, transparent mixtures of aromatic polycarbonates and polyalkyl methacrylates can be produced by melting the two components in the presence of a supercritical gas. Also, U.S. Pat. Nos. 4,743,654 and 4,745,029 disclose that one may produce solutions of the two polymers in organic solvents, allow the organic solvent to evaporate and thus produce a transparent material. Unfortunately all of these methods suffer from the drawback of bubble formation and other imperfections in the final product which would render them unsuitable for many applications. Since care must be taken in such processes, these methods most certainly would be relatively slow in comparison to traditional melt forming processes such as all forms of extrusion and molding and most likely would be limited to thin films as opposed to larger, more bulkier articles. A further disadvantage could be deterioration of properties and breakage arising from separation of the two phases since a blend produced by such processes would be metastable.
Accordingly the task existed of discovering and producing transparent PC/PMMA blends which exhibit all of the beneficial properties expected of such a mixture which is processable by known melt processing techniques.